1. Field of the Invention
The invention relates generally to attitude determination with global positioning system (GPS) satellite signals and more specifically to the resolution of carrier cycle integer ambiguities in the carrier phase difference measurement between separate GPS antennas attached to a rigid structure.
2. Description of the Prior Art
The United States Department of Defense has placed in orbit a group of satellites as part of a global positioning system (GPS) that can be used by civilians and the military alike to get automated and highly-accurate earth position coordinates on easy-to-read digital displays. Determining where you are has been a particular problem for seafarers for thousands of years. Now, GPS enables small sailboat owners and even combat soldiers to get their positions to within several meters using hand-held portable equipment.
GPS-based attitude determination offers significant cost savings in applications where inertial guidance has traditionally been the standard approach. Attitude is measured by differential measurements of GPS carrier phase between two or more antennas. Performance may be characterized in terms of accuracy and bandwidth, both being dependent on application specific parameters, such as the antenna spacing and the carrier-to-noise ratio.
Factors which limit performance are multipath, carrier-to-noise ratio, and integer resolution. Techniques are available for working around multipath and increasing the bandwidth of differential carrier phase tracking.
The rapid resolution of integer ambiguities in measured GPS carrier phase data is a principal obstacle in high performance systems. Integer ambiguity makes it difficult to determine the integer number of carrier cycles that occur between the antennas and the cable paths. For example, as shown in FIG. 1, a single carrier intersects two antennas. The phase angle at the first antenna is zero degrees, and the phase angle at the second antenna is 72 degrees. There may, however, be an additional full cycle between the antennas. As the signals travel their respective paths from the antennas to a pair of correlators, additional phase shifts appear. If the first antenna cable path is 3.6 wavelengths, and the second is 0.8 wavelengths, the correlator connected to the first antenna sees a signal sin (.omega.t-1296 degrees), while the correlator connected to the second antenna sees a signal sin (.omega.t+72+360-288 degrees). The signal correlators measure the first-difference carrier phase, the phasor difference between the signals seen at a channel one and a channel two correlator input. For this example, the basic output is a phase measurement of 288 degrees, or 0.8 wavelengths.